Top coating compositions to coat outer panels of automobiles are required to provide top coating film exhibiting color appearance of high grade. To meet this demand, effect pigment-containing base coating compositions are developed.
Base coating compositions in general are for forming underlayer part coating film where the top coating film is composed of multi-layers of coating films. By forming a multi-layered film by applying a transparent clear coating film on a base coating film formed of such a base coating composition, a top coating film exhibiting color appearance of high grade can be obtained, which has both excellent color appearance attributable to the base coating film and superb gloss and surface smoothness attributable to the clear coating film.
Effect pigment normally has laminar or flaky structure and is oriented in the coating film in parallel with the substrate surface to glitter brightly and change color tone according to viewing directions, whereby forming a coating film of unique color appearance. Of such properties, the one of changing the color tone according to change in viewing directions is referred to flip-flop property (FF property), greater changes in color tone signifying higher flip-flop property and better brilliance of the coating film containing the effect pigment.
Conventionally, organic solvent-based base coating compositions have been widely used for effect pigment-containing base coating compositions. Recently, however, water-borne base coating compositions causing less environmental pollution are increasingly adopted, due to environmental pollution with volatilization of the organic solvent during baking of applied coating film. Whereas, it is more difficult to obtain stable appearance with water-borne base coating compositions as compared with organic solvent-based base coating compositions, because of low volatilization rate of water, which is the diluent, from their coating films and, furthermore, because the volatilization rate is significantly affected by ambient application conditions, in particular, temperature and humidity. Thus, coating films of water-borne base coating compositions are subject to the problem that degradation in brilliance is apt to be invited by decrease in flip-flop property or occurrence of unevenness in metallic finish.
In industrial coating lines, normally the work is separately conducted for each zone using a same kind of coating composition, whereby to control degradation in coating quality caused by scattered coating composition's sticking on the substrate or coated film. For example, an automobile coating line is generally divided into undercoating zone, intermediate coating zone, base coating zone and clear coating zone.
Also within each of such coating zones, normally the coating operation is divided into two or more steps and a setting time of from about 30 seconds to 3 minutes is provided between the steps to prevent sagging of coating composition and secure high coating quality. Such coating steps within a same zone are referred to, by the order of being conducted, as the first stage, second stage, and so on.
In the recent years, as one of the means for obtaining coated articles with coating film of high brilliance using water-borne base coating compositions inducing less environmental pollution, a coating method is proposed in which different specific effect pigment-containing water-borne base coating compositions are used for the first and second stages at a base coating zone.
For example, JP2004-351389A discloses a coating film-forming method comprising applying a water-borne first brilliant base coating composition having a solid content of 10-45 mass % in the first stage of the base coating zone to form a first base coating film; and applying in the second stage a water-borne second brilliant base coating composition having a solid content of 10-40 mass % on the first base coating film to form a second base coating film, the ratio between the solid contents of the water-borne first brilliant base coating composition and the water-borne second brilliant base coating composition being 1.1/1-4/1. However, this coating film-forming method is subject to a problem that layer-mingling takes place at the interface of the coating films in the occasion of forming the second base coating film on the first base coating film at the second stage, presumably due to the difference in solid content of the two coating films, which disturbs orientation of the effect pigments in the vicinity of the interface and occasionally degrade the brilliance.
Also JP 2004-351390A discloses a coating film-forming method comprising forming a first base coating film with a water-borne first brilliant base coating composition in which the mass concentration of effect pigment in the composition is 1-30%, in the first stage of the base coating zone, and forming in the second stage a second base coating film with a water-borne second brilliant base coating composition in which the mass concentration of effect pigment in the composition is 5-40%, the ratio between the mass concentration of effect pigment in the water-borne first brilliant base coating composition and that in the water-borne second brilliant base coating composition being 1/4-1/1.1.
However, this coating film-forming method is subject to a problem that orientation of the effect pigment in the second base coating film is disturbed to reduce brilliance or the second base coating film tends to become relatively brittle and occasionally comes off, because of the high mass concentration of the effect pigment in the second base coating film.
Furthermore, JP2004-351391A discloses a coating film-forming method comprising forming a water-borne first base coating film with a water-borne first brilliant base coating composition in the first stage of the base coating zone, the mass ratio in the solvent of said composition being: organic solvent/water=5/95-49/51, said organic solvent containing 40-100 mass % of a specific organic solvent which has an evaporation rate of 150-800 (where the evaporation rate of n-butyl acetate at 25° C. is set to be 100) and a solubility parameter of 9.5-14.5, and then forming in the second stage a water-borne second base coating film with a water-borne second brilliant base coating composition. This coating film-forming method, however, is liable to induce environmental pollution because a larger amount of organic solvent must be used compared with ordinary water-borne base coating composition.